In cold weather, some cloud banks can contain supercooled water — water that's below freezing, but still in liquid form. When an airplane flies through a cloud like this, it leaves a temperature differential behind the jet or propeller. This temperature drop is often enough to form ice crystals in the supercooled cloud. These crystals then spread, and cause the cloud to release rain and snow.

The effect isn't enough to influence global climate, but what it can do is radically change the weather around a small area — like, say, an airport. According to the researchers, these clouds can be found within 100km of airports up to 5%-6% of the time, and substantially more as you head towards the poles.

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Maybe that explains why it's impossible to have an on-time flight in Chicago 75% of the time.